Ultra fine austenite grain/carbide size stainless

[Cliff Stamp]

Recently I posed some information about nano-structured steel, in particular bainite :

- viewtopic.php?f=2&t=66230" target="_blank

This is a growing area of research and the bainite has very different properties than conventional bainite. It retains the high toughness, but produces a hardness and wear resistance on par with martensite steels even able to match the abrasive wear resistance of D2.

The positive effect of ultra-fine grain structure is also used in martensite of course, it isn't just of benefit in bainite. In particular using ultra-fine grain sizes is utilize in the ultra-high carbon steels to produce very high levels of strength, toughness and wear resistance. I posted some information on that as well :

- viewtopic.php?f=2&t=65926" target="_blank

Now the interesting thing that I asked immediately in reading that is has anyone done it with stainless steels and would the same general technique work. It didn't seem to be obviously flawed reasoning that manipulating the microstructure by forming a very fine array of sub-micron carbide and then quenching from that state could also produce stainless steels with refined austenite grain size and carbide size.

As it turns out, yes this can be done, and in fact it was done decades ago. The place to turn it up, as Landes always argues is in the most expansive area for high performance cutting steels -razor blades-. The razor blade industry is huge and there is a wealth of data on it. Even small differences mean a lot when you are looking at the volume they make and so the patents on steels and processing are numerous.

This one :

https://docs.google.com/viewer?url=pate ... 180420.pdf" target="_blank

References a way to produce a very high hardness and very fine carbide structure using the same general procedure talked about previously where the initial state of the steel is manipulated so that in the soak before the quench, a very fine austenite grain size is produced and does so without exposing the steel to high heat, and retains the necessary dissolution of the carbides into the martensite.

This allows toughness at very high strength (up to 64 HRC for a 0.45% carbon low alloy stainless steel), very high corrosion resistance and an extremely fine carbide structure. The size of the primary carbides is sub-micron, they are between 200 and 500 nanometers, or 0.2 to 0.5 microns. In comparison the carbides in a standard razor blade steel like AEB-L are 1-2 micron and in traditional steels such as ATS-34 and 440C they are 10 to 30 microns .

How do they do it

" (a) heated to 550 to 750° for about 5 to 30 minutes; (b) subjected to a temperature in the range from immediately above the Acl point to the Acl point plus 50° C. for 5 to 30 minutes to make the steel austenitic; (c) held at a temperature within the range just below the Acl point to the Acl point minus 80° C. for at least 10 minutes to complete the isothermal transformation; (d) subjected to a temperature between 600° to 650° C. for about 5 to 10 minutes and (e) then cooled, e.g. by air or a combination of air and water quenching to room temperature."

This is followed by the standard hardening for that type of steel :

" Generally the conditions under which the steel can be hardened and still retain the fine carbide structure are known to the art. In a preferred mode of carrying out the hardening step according to the processes of the present invention the steel is heated to a temperature between about 1000° to 1150° C., preferably to about 1075° to 1100° C. and held there for about 5 seconds to 60 seconds and then quenched to room temperature and preferably to a temperature between -20° C. to -120° C. and usually to a temperature between -40° C. to -80° C. "

It is a multi-quench method of hardening which uses the first quench to put the steel in a state which maximizes the effect of the second quench. The gains are noted to be significant, increasing the hardness by ~4 HRC points and as noted it allows the steel to easily compete in hardness with the 0.6% razor blade steels while maintaining the ease of working (blanking) of a 0.45% carbon steel.

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Now what does all of this mean with knives? Well at a basic level, for the high end knives which can do small volume heat treatment it opens up an opportunity for experimentation. Even for Spyderco it would be worth a discussion with a plant manager to see for example if the hardening of the standard steels they use could be subjected to a similar cycle which as noted can produce very significant results. Imagine for example if Spyderco could start making S30V at 64 HRC with an ultra-fine austenite/carbide grain structure which was not only harder and more wear resistant than the standard 60 HRC S30V, it would be as tough or tougher at the same time with improved corrosion resistance.

[Mike Blue]

I suspect that it is not economically feasible or industry would have adopted it years ago. But then, maybe they have: https://youtu.be/e9PnTPIKd3g" target="_blank It seems there are quite a few similarities in the method described on "How It's Made" and your criteria for temperatures. They don't identify the steel except to call it stainless.

Bainitic structure is what I like in the stuff I make. I can't argue with a desirable characteristic. If industry could do this cheaply and save money on materials...

[Cliff Stamp]

What they describe is the standard hardening, the critical part is the patent information above is the processing before that, to cycle the quenching twice with the first cycle not being done to produce a specific hardness but a specific carbide structure to maximize the effect of the second quench.

I think bainite is under explored in steels in cutlery, especially with all the discussions of very high toughness and the manufacturing of knives which are used in very heavy work and where chipping/fracture becomes a critical mode of failure.

[noseoil]

I'm wondering about this type of heat treatment & the thickness of the material type being used. Is there an upper limit to thickness and would a blade fall into this category?

Since it's a process, it can be applied to different types of materials, but will the market support this cost basis for a knife blade? It would be wonderful to have a high-hardness blade like this, which exhibits these enhanced properties, but what would it cost to manufacture? Since a "normal" blade is already heat treated and these costs are factored into the basic production, it sounds like a slight increase in cost might be justified in the market place, especially to justify a superior product.....

[Cliff Stamp]

To be clear, lots of makers do more than one cycle in the heat treatment, it is very common among ABS guys to multiple quench or multiple normalize. I have not seen any manufacturer talk about it but again all you are talking about is simply running two heat treatment cycles. A standard heat treatment, using one of the well known companies, can cost as little as $10 a blade. The main reason this is discussed in the literature is because it is so inexpensive compared to even changes such as 420HC to AEB-L.